Covering material for agricultural and horticultural soil

ABSTRACT

A covering material for agricultural soil contains starch or a derivative or fraction thereof as binder together with a structure-improving amount of an acid and/or an oil, and a finely divided vegetable filler such as straw. The oil is for instance palm oil or linseed oil in an amount of 1-25 wt. %. The acid is preferably 0.2-4% by weight of phosphoric acid or citric acid. The material may further comprise fungicides, bentonite and/or rubber latices.

This application has been filed under 35 USC 371 as the national stageof international application PCT/NL95/00315 filed Sep. 22, 1995.

FIELD OF THE INVENTION

The invention relates to a weed-inhibiting covering material, foragricultural soil, which contains a polymer carbohydrate and a vegetablefiller.

BACKGROUND OF THE INVENTION

A covering material of this type is disclosed in Netherlands PatentApplication 8402655. The known material consists of a suspension of, forexample, cellulose waste (paper) as filler, carboxymethylcellulose orpolyvinyl acetate as binder and, in addition, a light-absorbent agent,such as soot, and inhibits the growth of grasses.

According to Swiss Patent 678478, ground in parks and gardens can beprotected against weeds by applying a layer of material consisting offelted paper residues, optionally mixed with polystyrene foam.Netherlands patents 91383 and 98116 teach the use of hydroxyalkyl andcarboxymethyl starches and swelling starch, respectively, for protectingsoil against erosion.

However, the protective effect of the known covering materials is ofrelatively short duration. Furthermore, these materials also containpoorly degradable constituents (polyvinyl acetate, polystyrene) andmould formation often takes place in the material in the course of time.

SUMMARY OF THE INVENTION

A covering material has now been found which is inexpensive, provideslong-lasting (many months) protection against the growth of weeds,protects the soil against erosion and, furthermore, can be appliedeasily and is completely biodegradable.

The covering material, for agricultural soil, according to the inventiontherefore contains a polymer carbohydrate as binder and a finely dividedvegetable filler and is characterized in that the binder is starch or afraction or derivative thereof together with a structure-improvingamount of an acid and/or an oil.

Surprisingly it has been found that water uptake is lower, even at highRH (relative atmospheric humidity) and, partly as a result of this, theprotective effect of the covering material is appreciably better whenthe polymer carbohydrate used is starch or a fraction or derivativethereof with improved structure. The covering material according to theinvention is effective in controlling grasses, weeds, mosses and otherundesirable plants, reduces erosion, saves irrigation water and may alsopromote plant growth.

In this context, agricultural soil is understood to be any soil on whichcrops can be cultivated, either in the field outdoors or in greenhouses,optionally in trays or pots. Agricultural soil therefore also includes,for example, horticultural soil, (tree) nursery soil, garden soil andpotting compost. The soil concerned is in the main natural, optionallycultivated vegetable compost, but also completely or partially syntheticcompost.

DETAILED DESCRIPTION OF THE INVENTION

The polymer carbohydrate binder can consist of starch or a fraction orderivative thereof, for at least 50 wt. % or even at least 75 wt. %.Fractions and derivatives are understood to be decomposition productssuch as amylose and amylopectin and partially depolymerized products,and physically and chemically modified products, such as extruded,dried, oxidized, hydroxyalkylated, carboxymethylated and acetylatedstarches, and depolymerized, crystallized starches and other forms.Carboxymethyl-starch, native (unmodified) starch, extruded starch and inparticular roller-dried starch, which can be converted to a starchsolution in cold water and can be obtained, inter alia, under the nameFlocgel lvw, have been found to be suitable, amongst others. A polymercarbohydrate means here a carbohydrate having at least 15, especially atleast 25 monosaccharide units. In addition to the starch material, thebinder may also comprise cellulose and hemicellulose, and derivativesand fractions thereof.

In particular, the structure of the starch material can be improved andthe drying thereof can be promoted by adding an oil to it. The term oilshould be understood to comprise glycerol esters, in particulartriglycerides of medium and long-chain fatty acids. It comprises fatsand oils which are liquid at ambient temperature, but also fats thathave higher melting points, such as between ambient temperature andabout 75° C.

The oil is advantageously a vegetable oil. Vegetable oils which can beused are, for example, linseed oil, rape oil, sunflower oil, coconutoil, palm oil and palm kernel oil. The oil preferably has a meltingrange above ambient temperature, and in particular between 25° C. and50° C. Palm oil is the most preferred because it has a favourablemelting range and is hydrolyzed to only a slight extent. Linseed oil isanother preferred oil. Mixtures of oils can also be used. The amount ofoil to be used is, for example, 1-25% by weight, preferably 5-20% byweight and particularly preferentially 8-15% by weight, based on the drycovering material. The oil can be added by simple mixing, after meltingif applicable. The oil can also be added by extrusion, e.g. if it is asolid fat.

Alternatively, or in combination, the structure and the weed-resistanteffect of the starch material can be improved by the addition of acid.The use of an acid is especially preferred under relatively highhumidity conditions. The acid used in the covering material according tothe invention is an acid which does not leach out quickly. Examples ofsuch acids are phosphoric acid, oxalic acid, succinic acid, lactic acid,malic acid, benzoic acid, citric acid, gluconic acid, and the like, thepreference being for citric acid and phosphoric acid, and the greatestpreference being for phosphoric acid. Combinations with these and otheracids are also readily possible. The total amount of acid is preferably0.2-4% by weight, in particular 1-1.5% by weight, based on the entiredry mixture.

Suitable fillers are virtually all finely divided vegetable fillers,such as chaff, straw, hay, hemp, paper and wood. The particle size ofthe filler is found to be important. If the particles are too large, thelayer of covering material becomes too thick and can be more difficultto apply. The particle size is preferably such that the filler passesthrough a sieve with a mesh size of 2.5 mm and more preferentiallypasses through a 1 mm sieve. However, the particles must also not be toosmall, because the drying properties of the filler then becomeinadequate. Preferably the particles do not pass through a 0.1 mm sieve,and in particular do not pass through a 0.25 mm sieve. Outstandingresults have been recorded with finely divided straw having an averageparticle size of 0.5-1 mm. The vegetable filler can be crushed in anydesired way, for example using a hammer mill, pin mill or cutter mill.In the case of straw, the best results are obtained with a hammer mill.

The weight ratio of filler to binder can vary from, for example, 10:1 to1:2, in particular from 4:1 to 1:1, based on solids.

It was found that the performance of the covering material can befurther improved by incorporation of a clay mineral. Suitable clayminerals include e.g. bentonite. The clay mineral may be added in anamount of 3-30%, especially 5-25% by weight, with respect to the totalweight of dry binder and filler material and optional oil. The claymineral causes the covering layer to be stronger and more resistant torain.

Furthermore it was found that the addition of a latex material improvesthe resistance of the covering material to rain and wear, whereas thebiodegradability was still fully satisfactory. The latex material ispreferably a natural latex such as rubber latex, but it may also be asynthetic latex if this is sufficiently degradable, such as apoly(hydroxyalkanoic acid). The latex can be added in an amount of2-30%, especially 4-20% by weight, with respect to the total weight ofdry binder and filler material and optional oil.

If necessary, the covering material can contain other constituents, forexample in order further to control undesired plant growth and/or mouldgrowth. For instance, the material can contain a light-absorbentsubstance, for example carbon, or a fungicide. In general a fungicidewill not be needed if the covering is used outdoors, in the open field,but will frequently be needed if the material is used in a greenhouse.Examples of acceptable and effective fungicides are imazalil (especiallyfor starch), azaconazole and propiconazole (especially for cellulose)and cycloheximide. Other possible additives are, for example,plasticisers such as urea or glycerol. All constituents of the coveringmaterial must be biodegradable, that is to say must have been completelydecomposed, under the influence of organisms present in the ground, intoharmless substances within a period of, for example, 6 months to a year.

The covering material can be prepared by mixing all dry constituents,optionally with the exception of the latex if used. The acid can beadded at this stage or at a later stage. At the location where thecovering layer is applied, the dry mixture can be mixed with a suitableamount of water or other solvent, such that an easily spreadablesuspension or slurry is obtained. In general, 5-10 parts of dry mixtureare introduced into 90-95 parts of water. The liquid constituents,including, if necessary, the acid and the latex, are added thereto andthe whole is mixed. The suspension is then spread over the soil using anamount of 100-1000 g/m² (10-100 kg/are), in particular 300-700 g/m², anddried slowly (for example over a period of 1-4 days). A robust layer 1-6mm thick is thus obtained, which remains intact for several (2-24)months and effectively inhibits weeds.

Any pouring, spraying or atomizing installation can be used forapplication of the slurry or suspension. One example is an atomizerwhich contains a disc which rotates about a vertical shaft, thesuspension being poured onto said disc and then being strewn in alldirections at high speed in atomized form by said disc and spreaduniformly over the soil.

EXAMPLE 1

A mixture of 58.7% by weight of finely divided chaff with an averageparticle size of 0.5-1 mm was mixed with 29.3% by weight of physicallydestructed potato starch (Flocgel lv) and 10.6% by weight of palm oil.The mixture was mixed with water in a weight ratio of 1:10.

The suspension was mechanically spread over cultivation soil in a ratioof 650 g/m² (dry matter) and after drying for about 3 days formed arobust layer of about 4 mm thick, which was not washed away by rain andprevented the growth of grasses and other undesirable plants for severalmonths.

EXAMPLE 2

A mixture was prepared according to example 1, to which furthermore 1.4%by weight (with respect to the total dry matter) of citric acid wasadded. The suspension formed a robust layer of about 4 mm thick afterabout 3 days and prevented the growth of grasses and other undesirableplants for several months including wet periods.

EXAMPLE 3

A mixture of 58.7% by weight of finely divided straw with an averageparticle size of 0.5-1 mm, 29.3% by weight roller-dried potato starch(locgel lvw) and 10.6% by weight palm oil was prepared. The mixture wasmixed with water in a weight ratio of 1:10.

The resulting suspension was mechanically sprayed in an amount of 500g/m² (solids) over compost and after drying for about three days formeda robust layer of about 3 mm thick which was not washed away even in adownpour and also prevented the growth of grasses and other undesirableplants for at least 1 season.

EXAMPLE 4

A mixture was prepared according to example 3, and 1.4% by weight (basedon the total dry mixture) of 85% phosphoric acid was added.

The suspension was dry within 3 days even at high atmospheric humidityand was about 3 mm thick after drying. It was not washed away bydownpour and prevented the growth of grasses and other undesirableplants for at least 1 season.

EXAMPLE 5

A mixture was prepared according to example 3, and 10% by weight (basedon the total dry mixture) of natural rubber latex was added. Afterapplication a dry layer of about 3 mm thick after drying. It was notwashed away by downpour and prevented the growth of grasses and otherundesirable plants for at least 1 season.

We claim:
 1. Weed-inhibiting covering material for agricultural soil,comprising:starch or a derivative or a fraction thereof as a binder; afinely divided vegetable filler having an average particle size of 0.25mm to 1 mm, and selected from the group consisting of straw, hemp, paperand chaff; and a structure-improving amount ranging from 5 to 20% byweight of palm oil.
 2. The weed-inhibiting covering material of claim 1,wherein the filler is straw.
 3. The weed-inhibiting covering material ofclaim 1, wherein the material also contains a fungicide.
 4. Theweed-inhibiting covering material of claim 1, wherein the material alsocontains 2-30% by weight of a natural rubber latex.
 5. Method forprotecting crops against weeds, wherein a covering material according toclaim 1 is applied in an amount of 100-1000 g/m², based on solids, tothe soil in which the crops are growing.
 6. Weed-inhibiting coveringmaterial for agricultural soil, comprising:starch or a derivative or afraction thereof as a binder; a finely divided vegetable filler havingan average particle size of 0.25 mm to 1 mm, and selected from the groupconsisting of straw, hemp, paper and chaff; and a structure-improvingamount ranging from 0.2 to 4% by weight of an acid selected from thegroup consisting of citric acid and phosphoric acid.
 7. Theweed-inhibiting covering material of claim 6, wherein the filler isstraw.
 8. The weed-inhibiting covering material of claim 6, wherein thematerial also contains a fungicide.
 9. The weed-inhibiting coveringmaterial of claim 6, wherein the material also contains 2-30% by weightof a natural rubber latex.
 10. Method for protecting crops againstweeds, wherein a covering material according to claim 6 is applied in anamount of 100-1000 g/m², based on solids, to the soil in which the cropsare growing.